Transmission system



Filed Aug. 10, 1938 INVENTOR a. a. BJORNSON 2% a .m%

ATTORNEY Patented Sept. 26, 1939 UNITED STATES PATENT OFFICE TRAN SMIS SION SYSTEM Application August 10,

Claims.

This invention relates to signal transmission systems and particularly to volume control circuits for signal transmission systems.

One object of the invention is to provide a transmission line with control circuits for gov- 5 erning the volume of the signals on the line to maintain the volume substantially constant that shall have a syllable detector circuit for increasing the range of operation of the control circuits without undue operation by noise.

Another object of the invention is to provide a transmission line with control circuits for raising and lowering the volume of the signals on the line to maintain the signal volume constant that shall have a circuit operated by syllables of speech to increase the gain when the volume of the signals is below the operating level of the control circuits.

Another object of the invention is to provide a transmission line with control circuits having sensitivity less than the noise on the transmission line for varying the gain of the signals on the line to maintain the volume constant and a syllable controlled circuit having higher sensitivity for raising the gain on the line when weak signals are on the line.

A further object of the invention is to provide a transmission line with control circuits and a syllable control circuit of the above indicated character that shall control the gain on the transmission line at very low volumes by means of the syllabic circuit and that shall govern the gain on the transmission line at close to normal volume by the control circuits.

In volume control circuits, for example, the volume control circuit disclosed in the application of B. G. Bjornson Serial No. 224,086 filed August 10, 1938, a large sudden decrease in the input volume to the volume control apparatus may interfere with the normal operation of the volume control apparatus. If the volume control apparatus is operating at low gain and there is a sudden decrease in the input volume to the volume control apparatus then it is possible that no operation of the volume control apparatus will take place.

In accordance with the present invention a syllabic control circuit is provided for effecting gain on the transmission line when the input volume to the volume control apparatus is very low. The

" 7 syllabic control circuit is more sensitive for weak 1938, Serial No. 224,088

high because operation by noise may then take place. The sensitivity of the syllabic control circuit may be made high without danger of operation by noise by reason of the noise discriminating feature thereof.

The volume control circuits employed to describe the invention are disclosed in the abovementioned application of B. G. Bjornson Serial No. 224,086 filed August 10, 1938. A signal transmission line is provided with a variable gain device to control the gain so that two energy levels in speech E1 and E2 are exceeded only t1 and t2 per cent of the time. A balancing condenser is charged by potentials of opposite polarity under control of two detector circuits which are connected to the transmission line beyond the gain control device therein in any suitable manner, as by means of a hybrid coil. Each detector circuit comprises a marginal space discharge device which may be in the form of a three-element gasfilled tube. One of the marginal devices is set to operate or trigger oif when the energy level E1 and the other marginal device is set to operate when an energy level E2 is on the transmission line. Two relays are respectively operated by the two marginal devices. One of the relays, which is operated by a detector device when an energy level of E1 obtains on the transmission line, completes a circuit for charging the balancing condenser by potential of one polarity from a suitable source. The other relay which is operated by the other detector device when an energy level of E2 obtains on the transmission line completes a circuit by charging the balancing condenser by potential of opposite polarity from asuitable source.

When the charge on the balancing condenser is raised above a predetermined value by a charge of either polarity a transfer relay is operated to change the charge on a control condenser. The control condenser governs the gain control device in the transmission line. If the balancing condenser is charged with one polarity under control of the relay operated by the high voltage V1 then the transfer relay will be operated to impress a negative charge on the control condenser to increase the loss effect by the gain control devices. If the balancing condenser is charged above the predetermined value with opposite polarity under control of the relay operated by the lower voltageVz then the transfer relay will be operated to discharge a portion of the negative charge on the control condenser to decrease the loss efiected by the gain control device.

Conditions may obtain on the transmission line so that the signals beyond the gain control device do not even have the lower voltage V2. The two detector circuits are backward-acting and are connected to the transmission line at a point beyond the gain control device. Low signal voltage may be on the transmission line beyond the gain control device if a very Weak talker follows a strong talker who has produced a very low gain.

According to the present invention a forwardacting syllable frequency circuit is provided for raising the gain when the energy of the signals is too low to successfully operate the normal volume control circuits. The syllabic detector circuit comprises an amplifier connected to the transmission line ahead of the gain control device, a detector connected to the amplifier and a filter or a tuned circuit which is connected to the detector for selecting impulses of syllabic frequency. A relay having the operating coil thereof connected to the output of the filter is operated by the syllabic frequency impulses. The syliabic frequency relay controls a second relay which has a predetermined hangover. The

1 second relay controls the relay operated by the V2 voltage on the transmission line and effects charging of the balancing condenser to increase the gain on the transmission line.

The syllabic frequency circuit is relatively ina sensitive to noise and therefore may have a higher sensitivity than the control circuits which normally control the gain control device to maintain constant volume on the transmission line. The syllabic frequency circuit, however, is sloweracting and once the volume is raised within 5 U decibels, for example, of a normal volume, the

normal control circuits will take control and maintain the volume substantially constant.

The single figure in the accompanying drawing is a diagrammatic View of control circuits constructed in accordance with the invention.

Referring to the drawing, a signal transmission line I having input conductors 2 and 3 and output conductors 4 and 5 is provided with a variorepeater B in the form of two space discharge devices 1 and 8. The device 1 is shown in the form of a pentode and is provided with a cathode 9, a control grid I9 and an anode l l. The device 8 is in the form of a pentode having a cathode 62, a control grid l3 and an anode [4. The variorepeater 5 is effectively connected in series with the transmission line i and is connected to the input conductors 2 and 3 by a transformer H5. The variorepeater is connected to the output conductors 4 and 5 by means of a transformer IS. A fixed biasing potential for the grids l and I3 of the pentodes l and 8 is supplied by a battery H. A variable biasing potential for the control grids of the pentodes l and 8 is supplied by a control condenser H3. The charge on the control condenser 18 is governed in a manner to be hereinafter set forth for controlling the variorepeater to govern the volume of the signals on the transmission line. A battery I9 is connected to the anodes of the devices 1 and 3 through resistance elements 20 and 2l.

Two detector circuits 22 and 23 are connected to the output conductors 4 and 5 of the transmission line by means of a hybrid coil 24. The

- detector circuit 22 comprises a three-element gasfilled tube 25. The input circuit of the tube 25 is connected to the hybrid coil 24 by means of a transformer 26. The output circuit of the tube 25 is connected in series with a coil 21 of a polarized relay 28. A condenser 29 associated with a resistance element 30 and an inductance eleto the battery 32.

ment 3| is provided to insure a hangover in the operation of the relay 28 by the tube 25. The hangover effected by the condenser resistance arrangement is preferably of the order .02 of a second. A battery 32 is provided for supplying plate potential to the anode of the tube 25. A battery 33 is provided for supplying filament heating current and grid bias to the tube 25.

The relay 28 is provided with a biasing winding 34 which is connected through a resistance 35 An armature 36 of the relay 28 which is operated by the coil 21 engages contact members 3? and 38. In the release position when the coil 2'! of the polar relay is not energized, the armature 35 is held in engagement with the contact member 38.

The detector circuit 23 comprises a threeelement gas-filled tube 39 which is marginal in operation and an amplifier tube 40 of any suitable type. The input circuit of the amplifier 40 is connected by a transformer 4| to the hybrid coil 24. The output circuit of the tube 40 is connected by a transformer 42 to the input circuit of the detector tube 39. The output circuit of the detector tube 39 is connected in series with the operating winding 43 of a polarized relay 44. A condenser 45 associated with a resistance element 46 and an inductance element 41 is provided for insuring a hangover in the operation of the relay 44 by the detector tube 39. The hangover time is of the order of .02 of a second. A battery 43 is provided for supplying anode potential to the detector tube 39 and for energizing a biasing winding 49 of the relay 44. The biasing winding 49 is connected to the battery 48 in series with a resistance element 50. A battery 5| supplies anode potential to the amplifier tube 49. A battery 52 supplies filament heat current and grid biasing to the detector tube 39 and the amplifier tube 40.

The relay 44 is provided with an armature 53 Which is adapted to engage contact members 54 and 55. When the operating coil 43 of the relay 44 is deenergized the armature 53 is held a of the order of 50,000 ohms and 130,000 ohms,

respectively. With the relay 28 in release position and the relay 44 in operative position, the balancing condenser 56 will be charged with potential from the battery 58 in circuit with the relatively large resistance 60. is in operative position the balancing condenser 56is charged from the battery 5"! in circuit with the relatively small resistance 59.

The detector circuits 22 and 23 are operated When the relay 28 when different energy levels of speech current are on the transmission line. The detector circuit 23 is assumed to be operated at a lower level,

for example minus 12 decibels, and the detector 22 is assumed to be operated at a higher level,

forexample, plus 10 decibels. The difference in r sensitivity, therefore, between the two detector circuits is 22.5 decibels.

A transfer relay 6|. which is controlled by the balancing condenser 56 serves to control the directionin which the charge on the control condenser I8 is changed. The transfer relay 6| comprises an operating winding 62, a biasing winding 63 and an armature 64 which is adapted to engage contact members 65 and 66. The operating winding 62 of the relay 6| is connected in the output circuit of an amplifier 61. The amplifier 67 is preferably in the form of a three-element space discharge device. Biasing potential for the grid of the amplifier 61 is supplied by the battery i559 and the balancing condenser 56. A battery 68 is provided for supplying anode potential to the amplifier 6? and for energizing the biasing winding 63 of the relay M. The circuit for the Winding 53 extends from one terminal of the battery 58 through the winding 63 and two resistance elements 69 and 16 in parallel to the other terminal of the battery 66. The resistance element 69 has a non-linear coefficientof resistance and preferably is composed of silicon carbide crystals and a binder material. Resistance elements of this type are disclosed in the patent to K. B. McEachron, 1,822,742, September 8, 1931. a

Normally with no charge of either polarity on the balancing condenser 56 the effects of the operating winding 62 and the neutralizing winding 63 for the relay 6| are neutralized and the armature 64 is held mid-way between the contact members 65 and 6'6. When the relay 28 is operated by the detector circuit 22 the condenser 56 is charged with potential of negative polarity from the battery 51. When the negative charge on the balancing condenser 56 is raised to a predetermined point the negative bias of the grid of the amplifier tube 6? reduces the plate current flow through the winding 62 of the relay 6i to move the armature 64 into engagement with the contact member 66. When the relay 44 is operated by the detector circuit 23 and the relay 23 is in released position, the balancing condenser 56 is charged with potential of positive polarity from the battery 58. Upon charging ofthe condenser 56 to a predetermined point by the battery 58 the positive potential bias on the grid of the amplifier device 6'! increases the plate current flow through the winding 62 of the relay 6] to move the armature '64 into engagement with the contact member 65. If the charge on the balancing condenser 56 is initially zero the armature 36 of the relay 28 will have to be left on the contact member 31 a definite length of time, for example .06 of a second, to allow the transfer relay 6I to move the armature 64 into engagement with the contact member 66. When the armature 64 engages :the contact member 66 the gain on the transmission line is reduced, as will be explained hereinafter. If the charge on the balancing condenser 56 is zero and the armature 56 of the relay 44 is moved into engagement with the contact member 55. it will take a definite length of time, for example .15 second, before the transfer relay armature 64 can be moved into engage= ment with contact member 65. When the armature 64 of the transfer relay 6| is moved into engagement with the contact member 65 the gain on the transmission line I is raised, as will be explained later.

A relay II comprising an operating coil I2 and an armature I3 is provided for insuring against change in the charge of the control condenser I8 except when speech currents are on the transmission line I. The coil I2 of the relay II is normally short-circuited by the armature 53 of relay 44 and the armature I3 is held in engagement with the back contact member I6. Upon operation of relay 44 the coil I2 of relay II is energized from battery I4 and the armature I3 is moved to engage a contact member I! for placing a ground on the armature 64 of the transfer relay 6|. Normally a shunt circuit is maintained around the battery I4 and the resistance element 15 to insure the release of the relay ii. The shunt circuit around the battery I4 and the resistance element I5 may be traced from one terminal of the resistance element I5 through contact member 54, armature 53 and ground to a terminal of the battery 14. Upon operation of the relay 44 the shunt connection around the battery I4 is opened so that the relay II is operated whenever the relay 44 is operated. The relay 44 is operated by the detector circuit 23 of higher sensitivity.

When the armature 64 of the transfer relay 8! is moved into engagement with contact member 65, the control condenser is discharged through a resistance I8. The circuit may be traced from one terminal of the control condenser I8 through the resistance element I8, contact member "65, armature 64, contact member Ii, armature I3 and ground return to the other terminal of the control condenser. When the armature 64 is moved into engagement with the contact member 66 the control condenser I8 is charged with potential of negative polarity from a battery I9. The charging circuit for the control condenser may be traced from one terminal of the condenser I8 through a resistance element 80, battery l9, contact member 66, armature 64, contact member Tl, armature I3 and ground return to the other terminal of the condenser IS. The size of the resistance elements 78 and 86.! may be varied as desired. The rate of gain increase depends upon the value of the condenser i8 and the resistance "I8. The rate of gain decrease depends upon the condenser I8, resistance element 86 and the battery "I9.

Assume that a weak talker transmits speech current over the transmission lineI with a low gain. Under this circumstance the relay 44 will be operated by the detector circuit 23 and the relay 28 will not be operated. After two or more syllables, depending upon the previous condition of the system and the input amplitude, the relay 6| will move the armature 64 into engagement with the contact member 65. When the relay 44 is operated the balancing condenser 56 is charged with potential of positive polarity from the battery 58. The circuit for charging the balancing condenser 56 may be traced from one terminal of the condenser through the armature 36 of the relay 28, contact member 38, resistance 66, battery 58, contact member 55 and armature 53 of the relay 44 to the other terminal of the condenser 56. The charge on the balancing condenser 56 impresses positive potential on the grid of the device '6'! to increase the current flow through the device. This effects operation of the transfer relay 6| tomove the armature 54 into engagement with the contact member 65. The negative charge on the control condenser I8 is reduced by a circuit including the resistance element I8. The relay II is operatedat this time inasmuch as this relay is operated whenever the relay 44 is'operated. A reduction in the negative bias upon the control grids I0 and it of the pentodes I and 8 increases the gain upon the transmission line I. It may be noted that the relay II which is operated only when the relay 44 is operated insures that change in the charge upon the control condenser I8 can only take place when speech currents are on the transmission line.

The circuits controlled by the relays 28 and 44 are timed so that when operated the balancing condenser 59 is charged with potential of opposite polarity and inversely proportional to the times of operation of the relays. The gain on the transmission line will be increased gradually until the relay 28 is operated. The relay 28 is operated when the energy level on the detector circuit 22 is raised above the higher predetermined energy level. Upon operation of the relay 28 the charging circuit of the balancing condenser 56 from the battery 58 is broken and a circuit is completed from battery 51 for reducing the positive charge on the condenser 56. When the condenser 55 is charged with potential of negative polarity to the critical point, negative'bias is impressed on the grid of the device 51 to reduce the current flow through the winding 62 of {the relay 6! sufficient to move the armature 64 into engagement with the contact member 66. At this time the relay II is in operation because the relay 44 is in operation. The relay 44 is always in operation when the relay 28 is operated. IIhe control condenser I8 is charged with potential of negative polarity from the battery I through the resistance element 80. Increase in the negative charge on the control condenser I8 increases the negative bias on the grids I0 and .I3 of the pentode tubes I and 9 to lower the gain on the transmission line I.

If the talker is talking at substantially constant volume it is possible to select the time constants of the circuits so that the critical charge on the balancing condenser 56 is never exceeded. If a critical charge of positive or negative polarity is never exceeded on the balancing condenser 55 the transfer relay 6I will not be operated and no change will take place in the gain of the transmission line.

A syllabic frequency detector circuit 90 is provided for controlling the relay 44 to raise the gain in case the voltage of the signals on the transmission line falls to a very low level. In case a strong talker is on the line the detector circuits 22 and 23 will set the variorepeater 6 for very low gain. If a very low talker is then put on the line the signals beyond the device 6 may not be able to operate the relay 44 by means of the detector circuit 23. The detector circuit 23 cannot be made with high sensitivity because of the danger of operation by noise when the variorepeater gain is high. The syllabic frequency detector circuit 90 has higher sensitivity than the detector circuit 23 except at high gain and is relatively insensitive to operation by noise on the transmission line.

The syllabic frequency detector circuit 90 comprises an amplifier 9|, of any suitable type, a detector 92 comprising space discharge devices 93 and 94, a filter 95 for selecting the impulses of syllabic frequency and three polar relays 95, 9'! and I25. The amplifier 9| is connected to the input conductors 2 and 3 of the transmission line I by means of a transformer 98, two condensers 99 and I00 and resistance elements IN and I02. The resistance elements IN and I02 and the condensers 99 and I00 in combination with the transformer 98 form a high impedance frequency selecinductance coil I04. A midpoint of the inductance coil I04 is connected to one terminal of the primary winding of the transformer I05. A grounded battery I06 is connected to the other terminal of the primary winding for the transformer I05to supply anode potential tothe devices 93 and 94. A by-pass condenser I0? is connected between the filaments of the devices 93 and 94 and the primary winding of the transformer I05. A condenser I08 shunted by resistance element I09 serves as a grid leak for the devices 93 and 94.

The relay 96 is provided with two windings H0 and III. The winding H0 is connected to the output circuit of the filter 95 which selects impulses of syllabic frequency. The winding III is energized from a battery I I2 to normally hold the armature I I3 in engagement with a back contact member II4. Upon energization of the winding IIO the armature H3 is moved into engagement with the front contact member I I5. The relay 91 comprises an operating winding I I6 and a biasing winding I H. The winding I I6 is energized from a battery I I8 upon operation of the relay 9%. The winding II! is energized by the battery IE2 and opposes the action of the winding I I5. Normally the winding II'I holds the armature IIO of the relay in engagement with contact member I20. Upon energization of the winding H6 the armature I I9 is moved into engagement with a contact member I2 I. A condenser I22 and two resistance elements I23 and I24 are included in the energizing circuit for the winding I It in order to provide a predetermined hangover in the release of the relay 91.

The relay I25 comprises an operating winding I29, a biasing winding I21 and an armature I23 which operates between contact members I29 and I30. The biasing winding I2! is energized by a battery I3I in circuit with a resistance element I40 to hold the armature I28 in release position and in engagement with the contact member I29.

The relay 96 when operated by energization of the winding IIO moves the armature H3 into engagement with the contact member I I5. A circuit is then completed from the battery IIS through the armature II3, contact member H5, resistance elements I23 and I24 and the coil H6 for operating the relay 9?. Upon release of the armature II3 the relay 9? is held in operative position for a predetermined time depending upon the time taken to charge the condenser I22 from the battery I I3. The condenser I22 is discharged whenever the armature H3 engages the contact member H5. The biasing winding I II which is energized from the battery I I2 holds the armature H3 in engagement with the contact member H4 whenever the winding I I0 is deenergized. Whenever strong syllable impulses operate the relay 95 the relay 91 may be held over between impulses. Thus the relay 96 may be operated by each impulse but the relay 97 may not be operated by each impulse.

The relay 91 when operated moves the armature I I9 into engagement with the contact member I2! for completing a circuit to energize the operating winding I26 of the pulsing relay I25. The circuit for energizing the winding I26 of the pulsing relay I25 may be traced from one terminal of the battery I3I through a condenser I32, armature H9, contact member I2I, winding I26 and resistance element I33 to the other terminal of the battery I3I. The pulsing relay I25 is operated for a predetermined length of time which is the time taken to charge the condenser I32 up to a certain value. Thus the relay I25 is operated a predetermined length of time for each operation of the relay 91 and irrespective of the strength of the syllable impulse on the transmitting channel I. Upon release of the armature II9 the condenser I32 is discharged rapidly through a resistance element I34. The biasing winding I2! of the pulsing relay I25 normally holds the armature I28 in engagement with contact member I 29. Upon energization of the operating winding I26 the armature I28 is moved into engagement with the contact member I30 for completing a circuit to energize the winding 43 of the relay 44. The circuit for energizing the winding 43 may be traced from grounded battery 48 through winding 43, resistance element I35, contact member I30, armature I28 and ground return to the battery 48. The relay 44 as above set forth controls the impressing of a positive charge on the balancing condenser 56 to effect operation of the transfer relay BI. The transfer relay 6| reduces the negative charge on the control condenser I8. Reducing the negative charge on the control condenser I 8 serves to increase the gain on the transmission line I.

Upon a sudden decrease in the input volume supplied to the variorepeater 6, the speech amplitudes supplied to the detector circuit 23 may be decreased to such an extent as to be unable to eiiect operation of the relay 44. However, at this time the syllabic frequency of the speech currents supplied to the syllabic frequency detector 95 will effect operation of the relay 96 which is connected to the filter 95. The relay 95 upon operation effects operation of the relay 9? and the relay 91 is held in operation for a predetermined time by reason of a hangover provided by the'condenser I22 and the associated resistance elements. The relay 91 upon operation effects operation of the pulsing relay I 25. The pulsing relay which is operated for a predetermined time effects operation of the relay 44. Upon operation of the relay 44 a predetermined number of times the balancing condenser 56 is charged sufficiently to effect operation of the transfer relay 6|. The transfer relay in turn controls the variorepeater to increase the gain on the transmission line. When the output volume of the transmission line is substantially correct the operation of the relay 44- is effectively controlled from the detector circuit 23.

The circuit completed by the relay 9? for operating the relay I 25 a predetermined length of time serves to insure that the pulsing relay is operated by a weak syllable impulse the same as for a strong syllable impulse. The syllabic frequency detector circuit 90 is not an essential circuit for the strong talker, but is a desirable circuit for the weak talker who produces weak syllable impulses. The hangover in the operation of the relay 91 is to reduce the number of operations of the pulsing relay I25 for the strong talker who produces strong syllabic frequency impulses. When the volume on the transmission line I increases the number of operations of the relay 96 increases until a certain maximum operation is effected and then it operates on practically every syllable. When syllable operation of the relay 95 takes place two operations of the relay 95 may follow each other very closely so that the hangover of the relay 9'! will prevent a separate operation of the relay 91 for each operation of the relay 95. Accordingly two syllables which cause two operations of the relay 96 may only cause one operation of the relay 9! and one oper- The result of the above operation is ain setting by the detector circuit 23 after adjustment near to constant volume had been effected. The operations of the relay 44 by the detector circuit 23 are so frequent that the relay 44 is usually operated before the syllabic frequency circuit 90 can take control. The syllabic frequency detector circuit usually loses control of the relay 44 when the gain on the transmission line is within 5 decibels of the correct value.

Modifications in the circuits and in the arrangement and location of parts may be made within the spirit and scope of the invention and such modifications are intended to be covered by the appended claims.

What is claimed is:

1. A signal transmission line having a gain varying device therein, control means governed by the signal amplitude variations on said line beyond said device for controlling the device by signal amplitude variations of predetermined value to maintain the signal volume substantially constant, auxiliary means controlled by signal variations of syllable frequency on the line before said device and having higher sensitivity than said control means for governing said device to raise the gain on said line and means for reducing the speed of gain change by said auxiliary means when a strong talker is on the line to effect gain changes by said control means.

2. A signal transmission line having a gain varying device therein, control means governed by the signals on said line beyond said device for controlling the device according -to amplitude variations of predetermined value to maintain the signal volume substantially constant, and auxiliary means controlled by signal variations of syllable frequency before said device for governing said device to raise the gain on said line, said auxiliary means being operated by amplitude variations of lower amplitude than the amplitudes of the variations required to operate said control means and effecting faster gain change by said device for a weak talker than for a strong talker.

3. A signal transmission line having a gain varying device therein, control means comprising two detector circuits of predetermined sensitivity governed by the signals on said line beyond said device for governing the device to maintain the signal volume substantially constant, and an auxiliary circuit connected to said line before said device and comprising detecting means, a filter selecting syllable frequency variations, and means having a higher sensitivity with respect to the signals on said line than said control means and governed according to the detected syllable frequency variations of said auxiliary circuit for governing said device to raise the gain on said line.

4. A signal transmission line having a gain varying device therein, control means governed by the signals on said line beyond said device for controlling the device according to amplitude variations of predetermined value to maintain the signal volume substantially constant, an

auxiliary circuit connected to said line before said device and comprising detecting means, a filter connected to said detecting means for selecting syllable frequency variations, and auxiliary means governed by said circuit for operating said device to raise the gain on said line when the amplitude of the signals is below the operating range of said control means, said auxiliary means effecting faster gain changes for a weak talker than for a strong talker.

5. A signal transmission line having a gain varying device therein, control means governed by voice currents on the line within a predetermined range for controlling said device to govern the volume of the voice currents on the line and a circuit connected to said line comprising detecting means, a filter selecting syllable frequency Variations from the output of said detecting means, relay means operated by the output from said filter, said relay means being operated separately by each speech syllable when voice currents on said line are below the operating range of said control means, means for reducing the number of operations of said relay means when the volume of the signals on the line is raised to the range of said control means, and means controlled by said relay means for operating said device to raise the gain on the transmission line.

6. A signal transmission line having a gain varying device therein, control means governed "by voice currents on the line beyond said device for controlling said device to maintain constant volume on the line beyond said device, detecting means operated by the voice currents on said line before said device, a filter for selecting the syllable frequency variation from the output of said detecting means, and auxiliary means operated by the output from said filter for operating said device to raise the gain on the line, said auxiliary means being operated when the volume of the voice currents on the line is below the range of operation of said control means.

'7. A signal transmission line having a gain varying device therein, two detector circuits connected to the line beyond said device and respectively operated by two different amplitude levels on the transmission line, control means governed by said detector circuits for governing said device to maintain the volume of the signals on the line substantially constant, and auxiliary means governed by volume variations of syllable frequency on the line before said device when the signals have potentials lower than the volume variation amplitudes necessary to operate said detector circuits for governing said device to raise the gain on said line.

8. A signal transmission line having a gain varying device there-in, two detector circuits connected to the line beyond said device and respectively operated by two different energy levels on the transmission line, gain increaser means controlled by one of said detector circuits for governing said device to increase the gain on said line, gain decreaser means controlled by the other one of said detector circuits for governing said device to decrease the gain on said line, and means comprising a syllabic amplifier detector circuit connected to said line before the device therein for controlling said gain increaser means to increase the gain on the line under control of the signal syllables when the signals on the line have potentials below that necessary to operate the two detector circuits.

9. A signal transmission line having a gain Varying device therein, a balancing condenser, control means for charging said condenser by potentials of different polarity according to the durations of two different energy levels in the signals on said line, means controlled according to the polarity of the charge on said condenser when charged above a predetermined level for governing said device to maintain the signals on the line at substantially constant volume and means controlled by syllable variations on the line when the signals have potentials below that necessary to operate said control means for charging said balancing condenser to effect raising of the gain on the line.

10. A signal transmission line having a gain varying device therein, control means governed by voice currents on the line for controlling said device to govern the volume of the voice currents on the line and a circuit connected to said line comprising detecting means, a filter selecting syllable frequency variations. from the output of said detecting means, a syllable relay operated according to the syllable variations in the filter output, a control relay operated by said syllable relay with a predetermined hangover, a

pulsing relay operated by said control relay and having a fixed time of operation, and means controlled by said pulsing relay for operating said device to raise the gain on said line.

BJORN G. BJORNSON. 

